Bee hotels to enhance native pollinators: A premature verdict?

Background/Question/Methods

Society is increasingly concerned with declining wild bee populations. Although most bees nest in the ground, considerable effort has centered on installing ‘bee hotels’ – also known as nest boxes or trap nests – which artificially aggregate nest sites of above ground nesting bees. Campaigns to ‘save the bees’ often promote these devices despite the absence of data indicating they have a positive effect. Here we investigate whether bee hotels do indeed support native pollinators rather than introduced ones or other organisms entirely. To do this we test six hypotheses using 200 bee hotels set up annually for each of three years in different urban green space types including residential and community gardens, city parks, and rooftop gardens through out the city of Toronto, Canada.

At the end of each field season, the bee hotels were collected, opened and each brood cell removed, individually labeled and placed in storage to overwinter at 4°C.  In April of the following year brood cells were moved to a sealed incubation chamber kept at 26°C and 60% humidity until adult emergence. They were then sexed and identified to species, permitting categorization of each individual as native or introduced to the study region.

Results/Conclusions

Introduced bees nested at 32.9% of sites and represented 24.6% of more than 27,000 total bees and wasps recorded (representing 47.1% of all bees recorded). The type of urban green space was a significant determinant of the abundance of native bees (greatest in residential gardens), introduced bees (greatest on rooftops and community gardens) and native wasps (greatest in urban parks), but not of introduced wasps. Native bees were parasitized more than introduced bees and females of introduced bee species provisioned nests with significantly more female larvae each year. Native wasps were significantly more abundant than both native and introduced bees and occupied 75% of all bee hotels each year.

Based on these findings we advocate for due diligence on the part of retailers and promoters of bee hotels to avoid “bee-washing”; that is, green-washing as applied to potentially misleading claims for augmentation of native and wild bee populations. Bee hotels are useful for ecological and behavioral studies, outreach in citizen science and pollinator education campaigns. However, to ensure “bee-washing” is minimized, more research is needed to elucidate the potential pitfalls and benefits of using them in the conservation of wild native bees.